1. Field Of The Invention
This invention relates to a nonvision sensor, and more particularly, to a tactile sensing transducer for converting tactile pressure into an electrical signal.
2. Background Of the Art
Tactile sensing involves the continuous, variable measuring of tactile force or pressure. In some respects tactile sensing for electromechanical devices is analogous to the human sense of touch in that information about the amount and distribution of tactile pressure over a surface can be received and transmitted. Not surprisingly, tactile sensing finds great utility in the field of robotics where the tactile sensors provide signals for negative feedback control of servomechanisms and the like. Tactile sensing can provide information about shape, texture, position, orientation, deformation, center of mass, and presence of torque or slippage with respect to an object in contact with the sensor. Other applications of tactile sensing will come to mind to those skilled in the art.
One of the methods employed in tactile sensing is the use of a medium whose electrical properties vary in response to pressure induced deformation. For example, some materials exhibit a piezoresistive effect, i.e. the electrical resistance of the material varies in response to deformation. Layers of such material sandwiched between two conductive plates will provide a means for detecting pressure when an electrical potential is established between the two plates, the current flowing between them will vary according to the deformation of the intermediate layer resulting from an external pressure forcing the plates closer together. The current will change according to Ohm's Law: ##EQU1## where I=electrical current
V=Voltage PA1 A=Area of conductive plates PA1 r=specific resistance, i.e. resistivity PA1 L=distance between plates PA1 R=Resistance; R=rL/A
Measuring the current can provide a means for measuring the tactile force applied to the plates.
The gauge factor of a resistive layer is a measure of the fractional change in resistance (R) divided by the fractional change in distance (L) and indicates the sensitivity of the resistive layer with respect to tactile pressure. Some materials used for the conductive intermediate layer also change in resistivity (r) in response to applied pressure.
Compression sensitive materials currently in use include, inter alia, foamed polymers which contain conductive fillers such as finely divided particles of metal or carbon. Polyurethane, and silicone are also commonly used.
One disadvantage of polymer systems in which the prefoamed polymer is mixed with conductive fillers and then blown to become a foam, is that such polymers lose conductive capability because the direction of the morphological expansion pulls the conductive particles apart.
The tactile sensor, or tactile sensing transducers can be configured with an array of electrodes to provide a measure of the distribution of tactile pressure over a surface. Ideally, the tactile sensor will have sufficient sensitivity, consistent reproducibility, and high resolution. In accordance with the present invention it is now possible to achieve these objects in an improved manner while avoiding the disadvantages of known systems.